The infrared spectral region is important for a number of reasons. The peak of the room temperature blackbody radiation spectrum is at about 10 μm. Thus, everything necessarily exchanges energy with the radiation field at infrared wavelengths and there is information on temperature, emissivity, etc. contained in the radiation field. At night, this radiation can be used to visualize the environment and to find particularly “hot” objects—such as, for example, people and engines. Because this makes the spectral region important for defense applications, the military has had a long standing focus on improved infrared technology. The atmosphere is somewhat transparent in two different infrared spectral windows (MWIR: mid-wave infrared 3-5 μm, and LWIR: long-wave infrared 8- to 12 μm), so these regions are of particular interest. Additionally, most molecular vibrations are in the infrared, the vibrational mode of the hydrogen molecule at 2.5 μm is the highest energy fundamental (as opposed to overtone) molecular vibration frequency. The very important C-H stretch vibration (important because all organic compounds have a signature in this wavelength region) is at around 3.3 μm, while heavier and more complex molecules have signatures at longer wavelengths. For example the P-O stretch, that is a signature of many nerve agents, is at around 10 μm. Thus, for many applications there is a need to monitor radiation across the infrared.
Both photon and thermal detectors are widely used to detect and image infrared radiation. Photon detectors absorb a photon and result in the creation of an electron-hole pair that is then collected (in a photovoltaic detector) or the change in resistance due to the increased carrier concentration is monitored (in a photoconductive detector). For thermal detectors the change in some property (for example, resistance, piezoelectric effect, etc.) with temperature is monitored. The sensitivity of all of these detectors is ultimately limited by several sources of noise. Cooled detectors (operating at very low temperatures, as low as liquid He ˜4.2° K.) can be made background limited, e.g. the signal to noise is limited by the background radiation that is present in the environment. Most infrared detectors—especially as the detector temperature is increased towards room temperature—are limited by various effects such as thermal generation of electrons and holes and junction leakage that generally scale with the area of the device. Thus, if the electromagnetic fields can be concentrated into a smaller area, such as by using an antenna structure, the detector's performance can be improved. The noise scales as the square root of the current and hence the area, so the figure of merit is the transmission T divided by the square root of the detector area Ad normalized to the antenna area Aa the figure of merit being T/√{square root over (Ad/Aa)}.
Despite the vast amount of work on antennas at RF and microwave frequencies, relatively little has been done on antennas for infrared frequencies. This is largely because the scale of the photon wavelength (˜several micrometers) is quite challenging and until relatively recently, available fabrication has not been up to the manufacturing challenge.
Thus, there is a need to overcome these and other problems of the prior art to provide antennas for electromagnetic detectors, in particular, infrared detectors, and methods for their manufacture.